The characters you can romance are Bly, Dena, Jolon, Kotori, and Tiva.
There are a few other characters with whom it’s possible to have a fling, but not an actual relationship. The ones I’m aware of are Inferi/Augury, Maker, Sonoma, and Winter.
I’ve never heard anything about a fling being possible with Onyx, and it definitely isn’t possible to romance him.